It is desirable to track the position and timing of aircraft rotor blades in order to maintain, adjust, and improve rotor blade operations. For example, knowing the position of helicopter blades permits the person maintaining the aircraft to make suitable adjustments to improve the rotor operations.
Known techniques for detecting the position and timing of helicopter rotor blades include, for example, the use of optical systems to create an image of a blade tip onto a sensor so that the moving blade will generate a sensor output that can be related to the blade position, and the use of two light sensors to measure the time it takes for a blade to pass between two divergent fields of view of the sensors to create timing and position reference signals.
One of the challenges of known techniques is that the measurement relies on light level contrasts between the blade and dynamic background light conditions, making it difficult to achieve accurate measurements during changing light conditions, for example during different times of day, various sun/cloud conditions, etc. Such systems can struggle in poor lighting conditions where there is little contrast between the blade and the sky such as a dark cloudy day and may not be able to detect the blade pass event. Active illumination can be used to improve the operations but most of these systems require the addition of a retro-reflective target to aid detection. Moreover, in situations where the sun has a direct path into the optics system, the light can either saturate the detectors or create false events, corrupting the timing measurements.
The difficulties can be exacerbated by differing light deflection and absorption characteristics attributable to different sets of rotor blades, for example due to age of the blades, materials, color schemes, blade coatings, and the like.